Audiometric Findings in Otosclerosis

(Scott-Brown's Otorhinolaryngology Head & Neck Surgery, 9th Edition - Chapter 89)

INTRODUCTION

1. PURE TONE AUDIOMETRY (PTA)

a) Air Conduction (AC)

• Shows elevated thresholds - the patient hears poorly via air conduction because the fixed stapes cannot transmit sound vibrations effectively to the inner ear.
• The hearing loss is typically bilateral (though often asymmetric), gradually progressive, and commences in early adulthood (2nd-4th decade).
• The loss is greatest at low frequencies initially (the classic low-frequency conductive pattern), gradually becoming flat as disease progresses.

b) Bone Conduction (BC)

• In early/uncomplicated otosclerosis, bone-conduction thresholds are normal or near-normal, reflecting intact cochlear function.
• The air-bone gap (ABG) is therefore present: AC thresholds are elevated while BC thresholds remain near 0 dB HL.

2. THE AIR-BONE GAP (ABG)

Calculating the ABG

• Measured as the difference between average AC and average BC thresholds.
• Three-frequency average over 0.5, 1 and 2 kHz is traditionally recommended, as the ABG is greatest at these frequencies.
• Some use a four-frequency average over 0.5, 1, 2 and 4 kHz, as the AC average more accurately reflects monaural hearing disability. The American Academy of Otolaryngology-Head and Neck Surgery recommends averaging over 0.5, 1, 2 and 3 kHz.

Pattern of the ABG

• The gap is characteristically greater at low frequencies (0.5-2 kHz), producing a "rising" pattern on audiogram (worse at low frequency, relatively better at high frequency).
• As stapes fixation becomes complete and severe, the ABG may extend across all frequencies.
• The maximum conductive loss from total stapes fixation is approximately 60-65 dB - hearing cannot be worse than this by a purely conductive mechanism alone.

Clinical Significance of ABG Size

• ABG ≥ 20 dB at 0.5, 1 and 2 kHz: traditional threshold to recommend stapes surgery.
• ABG 10-20 dB: experienced stapes surgeons may still operate, as patients appreciate even small gains especially in bilateral disease.
• ABG < 10 dB: some surgeons now operate even with a small gap, as the Carhart effect (see below) artificially depresses bone conduction thresholds, and the true ABG may be larger than measured.

3. THE CARHART NOTCH AND CARHART EFFECT

What is the Carhart Notch?

• A dip (notch) in the bone-conduction thresholds, classically at 2000 Hz (by approximately 15 dB), with smaller dips at 500 Hz (~5 dB), 1000 Hz (~10 dB), and 4000 Hz (~5 dB).
• It is a mechanical artifact - not a true loss of cochlear sensitivity, but a reduction in the efficiency of bone-conduction sound transmission caused by the fixed ossicular chain.

Mechanism - The Carhart Effect

• Route A: Directly through skull bone to the cochlea (osseous route)
• Route B: Via the tympanic membrane and ossicular chain to the cochlea (ossicular inertia route)
• Route C: Via air in the external auditory canal

Carhart Effect Values (from various studies - Table 89.2, Scott-Brown's):

Key Points about the Carhart Notch

• It is a mechanical/conductive phenomenon, NOT a sensorineural loss.
• After successful stapes surgery, the Carhart notch disappears - bone-conduction thresholds improve especially at 2 kHz, explaining the phenomenon of "overclosure" (post-op AC thresholds better than pre-op BC thresholds).
• Average improvement in BC over 0.5, 1 and 2 kHz after stapes surgery = at least 12 dB.
• The Carhart notch can be seen in any ossicular fixation, not just otosclerosis - so it is not pathognomonic.
• A recent literature review showed limited predictive value of the Carhart notch specifically for diagnosing otosclerosis.

4. TYMPANOMETRY

• Tympanometry shows a peaked, Type A tympanogram (normal middle ear pressure) because the tympanic membrane is normal and mobile, and the Eustachian tube functions normally.
• The compliance (admittance) in surgically confirmed otosclerosis is less than normal due to the stiffened ossicular chain, but the range overlaps so widely with normal that it does not aid diagnosis in an individual ear.
• The same limitation applies to multifrequency/multicomponent tympanometry.
• Tympanometry is useful to rule out otitis media with effusion (Type B/flat tympanogram) as an alternative cause of conductive hearing loss.

Key point: Tympanometry does NOT generally aid the diagnosis of clinical otosclerosis. - Scott-Brown's Vol 2, Chapter 89

5. ACOUSTIC REFLEX

• In otosclerosis with stapes fixation, the acoustic (stapedial) reflex is absent because the stapes footplate cannot move to generate the reflex response.
• This is important: an absent reflex in the presence of a conductive hearing loss with normal tympanogram strongly supports stapes fixation.
• In very early disease with minimal fixation, the reflex may still be present but with increased threshold.

6. SPEECH AUDIOMETRY

• Speech discrimination scores are typically well-preserved in uncomplicated (purely conductive) otosclerosis, because cochlear function is intact.
• The patient hears speech poorly due to the conductive block, but when sound is amplified to overcome the conductive component, word recognition is excellent (close to 100%).
• In the British National Study of Hearing, clinical otosclerosis was defined as: normal tympanic membrane + peaked tympanogram in normal pressure + ABG ≥ 15 dB at 0.5, 1 and 2 kHz.
• Poor speech discrimination (<30% pre-operatively) suggests significant cochlear involvement (sensorineural component) and is a predictor of poorer outcome from stapedotomy in far-advanced otosclerosis.

7. MIXED HEARING LOSS - COCHLEAR OTOSCLEROSIS

• BC thresholds are also elevated (above the ABG and beyond what the Carhart effect alone can explain).
• Two scenarios exist:
  1. Mixed impairment: conductive loss from stapes fixation + SNHL from cochlear involvement.
  2. Pure SNHL: rare - otosclerotic focus surrounds the cochlea without oval window involvement (very few documented cases in temporal bone studies).
• For accurate assessment of cochlear reserve, BC thresholds must be corrected for the Carhart effect before judging the degree of sensorineural impairment.
• Browning and Gatehouse found that on average otosclerotic patients have no worse BC thresholds than the general population; however, subsequent studies (Topsakal et al., Redfors and Moller) showed significantly greater SNHL in non-operated otosclerotic ears than age-matched controls.

8. FAR-ADVANCED OTOSCLEROSIS (FAO)

• Defined as average AC threshold > 85 dB HL with bone-conduction thresholds unmeasurable at the limits of audiometric equipment.
• Despite absent measurable BC, a significant conductive component may still be present (up to ~60 dB of latent ABG beneath the severe SNHL).
• Example: a patient may have 65 dB SNHL + 60 dB conductive loss = total AC ~125 dB (unmeasurable), yet after stapedectomy, AC improves to 65 dB HL (the residual SNHL).
• Audiological clues suggesting FAO rather than pure SNHL: prior audiograms showing ABG, family history of otosclerosis, previous stapes surgery, preserved (though reduced) speech discrimination, and CT evidence of cochlear otosclerosis.
• Pre-operative speech discrimination <30% and small ABG on CT suggest the patient will do poorly with stapedotomy alone and should be considered for cochlear implantation.

9. AUDIOLOGICAL ELIGIBILITY FOR SURGERY - SUMMARY

10. KEY AUDIOMETRIC POINTS - SUMMARY TABLE

CONCLUSION

1. Progressive conductive hearing loss with low-frequency predominance
2. Normal BC thresholds (corrected for Carhart effect)
3. Air-bone gap maximal at 0.5-2 kHz
4. Carhart notch at 2 kHz on BC audiogram (mechanical, not cochlear)
5. Type A tympanogram with absent acoustic reflex
6. Excellent speech discrimination (in pure conductive form)
7. Post-op resolution of Carhart notch confirming mechanical basis